1. Field of the Invention
The invention is directed to a pump power monitor method for gain control of an optical amplifier, and in particular, to a pump power monitor method for controlling the gain of an optical amplifier pumping at multiple, distinct wavelengths.
2. Technical Background
Raman amplification occurs when higher-energy or shorter wavelength, pump photons scatter off the vibrational modes of a materials lattice matrix, i.e. optical phonons, and coherently add to lower-energy or longer wavelengths, signal photons. Typically, a pump laser is used to provide pump radiation through a Raman medium to generate stokes radiation in another wavelength range by Raman scattering. The stokes radiation is then used to amplify a source signal conducted through the Raman medium. A direct consequence of this is that Raman amplification can be realized at any wavelength in any optical fiber by correct choice of the wavelength of the optical pump.
Interest in developing applications for Raman amplification subsided when erbium-doped fiber amplifiers and other rare earth-doped amplifiers became known. Erbium-doped fiber amplifiers typically require less power to generate a given amount of gain when compared to Raman amplifiers. However, erbium-doped amplifiers effectively operate over only a limited wavelength band. While an erbium-doped fiber can be used for amplification in a wavelength band extending from 1530 nm to 1610 nm, such an amplifier configuration would require at least three erbium-doped fibers to cover this entire range. In comparison, Raman amplification allows amplification of the entire wavelength range within a single optical medium.
Raman amplification has become a viable commercial technology with demonstrations of wave division multiplexing near the zero-dispersion wavelength of dispersion shifted fibers. In such applications, typically referred to as Raman-assisted transmission, a pump light is launched into an optical fiber at inline amplifier sights in an opposite direction to that of the source signal, or the signal being amplified. The amplification is distributed along the transmission fiber with gain increasing exponentially near the output end near of the transmission fiber.
As distributed Raman-assisted transmission is rapidly becoming a commercial reality, several technical problems must be overcome. In contrast to erbium-doped amplifiers, there is little control or knowledge of the result and gain prior to installation due to several variables including variation in the effective area of a single fiber or multiple fibers combined within a particular span, pump wavelength attenuation, including that of the fiber(s) themselves as well as between the fiber(s) and the amplifier, and Raman gain coefficient of the fibers that are combined to cover the span.
Specifically, due to manufacturing variations in the magnitude of Raman gain coefficient fiber, effective area attenuation at the pump wavelength, power optimization is not only necessary between fiber types but also within a particular fiber type. Another variable making control of gain and gain ripple difficult is the spectral variations within different fibers and particular fiber types. The spectral variations alone can cause gain ripple to be greater than 0.5 dB, and sometimes greater than 1dB. A further drawback is the manufacturing variability in the central pump power wavelength and thermal changes to the periodicity of the stabilizing fiber Bragg grating. Therefore, control of the amplification, including gain and gain ripple, within distributed Raman-assisted transmissions requires significant control, especially for transmission rates of 40 Gb/s for distances greater than 600 km.
This invention relates to a pump power monitor method for controlling the gain of an optical amplifier. More specifically, the present inventive pump power monitor method provides for gain control of an optical amplifier pumping at multiple wavelengths. Further, while the present inventive pump power monitor method is discussed with respect to Raman amplification, the method that may be implemented in conjunction with other optical amplifiers including, but not limited to, erbium-doped fiber amplifiers operating at multiple wavelengths.
In one embodiment, an optical fiber amplifier system includes an optical fiber adapted for use as an optical wave guide amplifier, and at least one optical pump optically coupled to the optical fiber, wherein the pump receives both a DC electrical input and an AC electrical input, and provides an optical pump power having both a DC optical power component and an AC optical power component to the optical fiber. The optical fiber amplifier system further includes a pump power detector optically coupled to the pump, and at least one controller connected to the pump power detector and adapted to determine the DC optical power component of the optical pump power, wherein the controller is adapted to adjust the DC electrical input to the pump.
In another embodiment, a Raman optical fiber amplifier system includes an optical fiber adapted for use as a Raman optical fiber amplifier, and at least one optical pump optically coupled to the optical fiber, wherein the pump receives both a DC electrical input and an AC electrical input, and provides an optical pump power having both a DC pump power component and an AC pump optical component to the optical fiber. The Raman optical fiber amplifier system further includes a pump power detector optically coupled to the pump, and a controller operatively connected to the pump power detector and adapted to determine the DC optical power component of the optical pump power, and adjust the DC electrical input of the pump.
In addition, embodiments of the optical fiber amplifier system include controlling the gain of a single optical amplifier operating at a given wavelength, controlling the gain of an optical amplifier that includes multiple pumps operating at multiple wavelengths, individual control circuits for controlling the gain of each of the optical pumps associated with the amplifier, and a switching system for controlling multiple optical pumps operating at multiple at multiple wavelengths with a single control circuit.
Other embodiments include an optical communication system that utilizes the pump power monitor scheme, as well as a method for utilization of the pump power monitor scheme.
Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings.
It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention which, together with their description serve to explain the principals and operation of the invention.
Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings.